BIRDS. 305 



how many ways their difference of structure and of colour 

 may cause them to be acted on "by the absorption or radiation 

 of heat, the action of light, or of electricity. Viewed merely 

 as a covering for the body, we find in aquatic birds a wise 

 provision to convert them into efficient non-conductors of heat, 

 by rendering them impervious to the water. Certain glands, 

 situated near the tail, secrete an oily matter, which is spread 

 by the bird over its feathers, and constantly renewed. By 

 this means the plumage remains unwet, even in the water; 

 and the stratum of air between the body of the bird and the 

 surface of the feathers being a bad conductor of heat, the vital 

 warmth of the body is not dissipated. Limiting our con- 

 sideration to another of their most obvious uses, let us view 

 them as portions of the wings. The feathers of the wing are 

 named according to the part 

 from which they have their 

 origin, and the bones are re- 

 garded as representing those 

 of the fore-leg of quadrupeds, 

 or the arm of man. Those 

 feathers that grow on the 



part which corresponds to Fig. 243. WING OF FALCON.* 



our hand are called the primaries (Fig. 243); those on what 

 may be called the fore-arm the secondaries; and those on the 

 part analogous to that between our elbow and our shoulder 

 (humerus) are named the tertiaries. 



Every one has noticed the quickness with which the wings 

 can be closed or expanded, and the compact space in which 

 they are shut up when not in use; but, regarded merely as a 

 piece of mechanism, their perfection is, perhaps, still better 

 evidenced by the number of hours during which they can con- 

 tinue in active operation, without fafi'gue to the bird by whose 

 exertions they are moved. The Swallow forms a good and 

 familiar illustration of this remark. During the time this bird 

 is employed in building its nest, or catering for its young, its 

 activity is ceaseless, and is interrupted only by the brief 

 intervals of rest atte ndant ontlie delivery of the material or 

 of the food. 



Perhaps the most striking illustration of long-sustained 

 powers of flight is afforded by the Frigate or Man-of-war- 



* p. primaries ; s, secondaries ; t, tertiaries. 



